Heterochrony in Middle Jurassic species of Gryphaea ...

Servicio Nacional de Geología y Minería, Sección Paleontología y EstratigrafíaCasilla 10465, correo 21, Santiago, Chile. E-mail: [email protected]

Heterochrony in Middle Jurassic species of Gryphaea(Ostreoidea, Gryphaeidae) from southern South America

Morphological changes along ontogeny, based on size, are analysed in five species of Gryphaea from the Mid-dle Jurassic of southern South America. Three of these taxa are new: G. apiculata n. sp., G. varillasensis n. sp.and G. euteicha n. sp. All five species have thin radial striae on the left valve. They are regarded as closelyrelated on the basis of the similar configuration of the main part of the left valve (especially during the firstontogenetic stages), the associated development of the posterior flange (and sulcus), the comparable variabilityand/or recurrent morphology of these and other structures, and their relative stratigraphic position or age. Theyshow differences in the relative appearance time and development of several characters, and similar changesprobably have also occurred stratigraphically, at least, in G. oxytropis PHILIPPI and G. euteicha n. sp. All thesefeatures allow the recognition of size (morphology)-based heterochronic processes (peramorphic and paedo-morphic), either in the origin of most of these taxa or in morphologic changes which developed along time insome of them. This work provides preliminary evidence of the most remarkable iterative occurrence of hete-rochrony known to date in oysters.

Heterochrony. Systematics. Oysters. Middle Jurassic. South America.

INTRODUCTION

The oysters are abundant in the diverse and well-pre-served macrobenthic fauna from the marine Jurassic innorthern Chile and mid-western Argentina. They have beenmentioned and analysed mainly from the middle of theXIXth to the beginning of the XXth centuries, based on speci-mens collected during geological surveys (Forbes, 1846;Hupé, 1854; Burmeister and Giebel, 1861; Gottsche, 1878,1925; Möricke, 1894; Tornquist, 1898; Jaworski, 1915,1925, 1926a, 1926b). The identified taxa, generally not illus-trated, were mainly related with European species. Further

detailed systematic studies were carried out by Gottsche(1878, 1925), Philippi (1899) and Jaworski (1915, 1925,1926a, 1926b), whose taxonomic conclusions influenced lat-er authors’ identifications of these oysters. Hayami (1961),Hallam and Gould (1975) and Hallam (1982) included someof the southern South American taxa in their analysis of theprobable interrelations of the Jurassic species of Gryphaea.Aberhan (1994) figured and characterized new Early Juras-sic specimens collected in northern Chile.

The Jurassic (and Triassic) oysters of southern SouthAmerica were recently studied in detail by Rubilar

Geologica Acta, Vol .3 , Nº2, 2005, 185-203

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© UB-ICTJA 185

KEYWORDS

A B S T R A C T

A. RUBILAR

(1998), and the main results were summarized in a pre-liminary paper (Rubilar, 2000). In this study, a group offive Middle Jurassic (and probably early Upper Jurassic)Gryphaea species was recognized. These species not onlyhave close morphological relationships (further than thepresence of thin radial striae on the left valve; see below),but also show clear anatomical differences when theirontogeny is compared. These differences also occurredalong the stratigraphical succession in some of them.

The aim of this contribution is to assess the ontogeneticaffinities and differences among these species in the lightof several heterochronic processes, based on material fromfive main localities in Chile and Argentina (Fig. 1).

Heterochrony in oysters: examples and criteriafor its recognition

The most-known example of heterochrony in oys-ters of the family Gryphaeidae involves three speciesof Lower Jurassic Gryphaea from England, in which apaedomorphic process has been recognized (Hallam,1982; Johnson, 1993, 1994; Jones and Gould, 1999;Gould, 2000). Another case corresponds to the proba-ble paedomorphic origin of Middle Jurassic ‘Catinula‘from Gryphaea in Europe (Stenzel, 1971; Hallam andGould, 1975; Sylvester-Bradley, 1977), although thegradualist pattern involved has been emphasized (John-son and Lennon, 1990; Johnson, 1993, 1999). Further-more, new and diverse examples have been found insome Tithonian and Neocomian oysters from themid-western Argentina (Rubilar et al., 2000).

The evidences advocated here to regard speciesrelated to each other in a phylogenetic sense, crucial indetermining heterochronic changes among taxa, aredescriptive: the degree of morphological (ontogenetic)affinities (Fig. 2; Table 1) and relative stratigraphicalposition. Nevertheless, the lack of phylogenetic recon-struction (e.g. using cladistic methods) is not a limita-tion to obtain significant heterochronic information(McKinney and McNamara, 1991, p. 31).

The heterochronic processes were inferred from rel-ative changes in size and shape of the specimens andtheir characters in a morphological sense (Table 1; Fig.3), and the terms of this size (morphology)-based hete-rochrony (‘allometric heterochrony’) follow McNama-ra (1986), McKinney (1988), and McKinney andMcNamara (1991).

These results are preliminary, because they shouldbe tested considering the ontogenetic age of speci-mens, necessary to confidently discuss the types ofheterochrony (‘true’ heterochrony) involved in changesamong species (Alberch et al., 1979; McKinney and

McNamara, 1991; Jones and Gould, 1999). Neverthe-less, classical studies of heterochrony are not compar-isons of shifts on developmental timing but allometricor morphological studies instead, and in several casessize seems to be a suitable proxy for age (McKinneyand McNamara, 1991; Smith, 2001). Furthermore, arecent approach re-focuses heterochrony as the changein the sequence of events, suggesting comparisonsacross taxa independently of external criteria (age,stage or size; Smith, 2001).

SYSTEMATICS, MATERIAL AND LOCALITIES

Of the five species here discussed, only Gryphaeaoxytropis PHILIPPI (1899, Lám. 5, fig. 1a, b) was previ-ously described. The holotype is housed in the MuseoNacional de Historia Natural of Santiago, Chile(MNHN, SGO PI. 5015). Three other supposedly dif-ferent taxa named by Philippi (1899) can be includedin this taxon: G. carinata PHILIPPI, G. tricarinataPHILIPPI and G. schultzei PHILIPPI. They have a moder-ate to large size, a nearly narrow main convexity zone(generally named ‘keel’), and a variable developmentof the anterior margin or flange in the main part of theshell (Fig. 2; Rubilar, 1998). In the context of thispaper, they are regarded as morphotypes in a probableanagenetic evolutionary series.

Three of the studied species are new (G. apiculatan. sp., G. varillasensis n. sp. and G. euteicha n. sp.).Due to the mainly paleobiological appeal of this contri-bution, their systematic treatment is restricted to thediagnosis and a general comparison among them (seeAPPENDIX). Another taxon (Gryphaea n. sp. A) is leftin open nomenclature considering the scarce materialavailable.

The thin radial striae on the left valve are also presentin other undescribed late Middle Jurassic oyster speciesfrom South America, generally assigned to the EuropeanGryphaea calceola QUENSTEDT, following Gottsche(1878, 1925). Nevertheless, they are characterized espe-cially by the development of, at least, three equivalentmorphotypes in the left valve (Rubilar, 1998), and proba-bly branched from an ancestral stock represented by theearliest G. euteicha n. sp. The taxonomic significance ofthis ornamentation is under evaluation.

Philippi (1860) described ‘Ostrea striata’ referringto the type of ornamentation mentioned above. Thegeographic and stratigraphic provenance of the typespecimen, lost in the MNHN, is uncertain in the northof Chile. It is only known from an external view of aleft valve in an early growth-stage (Philippi, 1860,Lám. 1, Fig. 10; 1899, Lám. 6, Fig. 3), which can be

Heterochrony in Middle Jurassic species of Gryphaea in South AmericaA. RUBILAR

186Geolog ica Acta , Vo l .3 , Nº2, 2005, 185-203

related with G. oxytropis and G. varillasensis n. sp.Nevertheless, its certain affinities with one or anotherspecies cannot be established at this stage, based on theontogenetic changes described here. Then, ‘Ostrea stri-ata’ is considered a nomen dubium.

The new data obtained from southern South Americanoysters lead to question the validity of the subgenus‘Bilobissa’ STENZEL (1971), which is therefore not adop-ted here and so the species dealt with are referred only toGryphaea in its wide sense.

Material

The taxonomical analysis was based on the followingmaterial, mostly left valves: 25 specimens of G. apiculatan. sp. (SNGM 8581-8605); 106 specimens of G. oxytropisPHILIPPI (SNGM 8364-8386, 8388-8434, 8437-8439, 8441-8464, 8466, 8475-8476, CPBA 18113, 18146-18150); 7specimens of G. varillasensis n. sp. (SNGM 8467-8473);118 specimens of G. euteicha n. sp. (SNGM 8435-8436,8440, 8606-8721); and 4 specimens of Gryphaea n. sp. A(SNGM 8722-8723; DNGM 7888, 7943).


187Geolog ica Acta , Vo l .3 , N º2, 2005, 185-203

FIGURE 1 Sketch showing localities mentioned in text, which are concentrated especially in two areas: A) northeastern Copiapó, Chile (thesynthetic stratigraphic log was measured in Quebrada Las Varillas). B) southwestern San Juan, Argentina. 1: Quebrada Palo Blanco; 2: Que-brada El Asiento; 3: Quebrada Las Varillas; 4: Quebrada Atacameño; 5: Arroyo Las Garzas; 6: Paso del Espinacito.

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The material described, illustrated or mentioned inthis paper is housed in the following institutions: ServicioNacional de Geología y Minería (SNGM, Chile), MuseoNacional de Historia Natural (MNHN, SGO PI, Chile),Instituto de Geología y Recursos Minerales (DNGM,Argentina), and Universidad de Buenos Aires (CPBA,Argentina).

Localities and age

Gryphaea apiculata n. sp. is recorded only from Que-brada El Asiento (Fig. 1A, locality 1), a locality describedby Pérez (1982, p. 43, fossil locality 11). It was collectedfrom Middle Bajocian beds, associated with a species ofStephanoceras ammonoid.

The here analysed specimens of G. varillasensis n. sp.and most of those of the G. oxytropis and G. euteicha n.sp. come from a short stratigraphical sequence located inQuebrada Las Varillas (Fig. 1A, locality 3).

In general, G. oxytropis is recorded from several locali-ties in Chile and Argentina (Rubilar, 1998), usually associat-ed with Reineckeiidae ammonites ranging from LateBathonian to Early Callovian in age (e.g. Riccardi and West-ermann, 1991; Riccardi et al., 1999). However, some speci-mens have been collected from certain Middle Callovianoutcrops in Argentina (P. Álvarez, oral comm., 2003).

In Quebrada Las Varillas, the first representatives ofG. oxytropis and G. varillasensis n. sp. are associatedwith the ammonites Chondroceras cf. allani (MCLEARN),Iniskinites (?) sp. and an indeterminate Perisphinctidae(Fig. 1A, level V1), which indicate a Late Early Bajocian– Late Bathonian age (A. Riccardi, oral comm., 1997).G. oxytropis is also present in other three probably LateBathonian levels of this succession (see below), whereasG. varillasensis n. sp. is absent from the last one (Fig. 1A,levels V1 to V3).

G. euteicha n. sp. was recovered especially from theupper level of the Quebrada Las Varillas section (Fig. 1A,level V4), associated with Late Bathonian Iniskinites andCadomites ammonoids. Nevertheless, it is also presentlower (Fig. 1A, level V3). On the other hand, in QuebradaEl Asiento (Fig. 1A) this species appears in stratigraphi-cal levels with Callovian Macrocephalites and Oxyceritesammonites (west of Estación Montandón; Pérez, 1982, p.59-60).

The biggest (‘giant’) Lower Callovian specimen of G.oxytropis ever known, collected by Álvarez (1996) inArroyo Las Garzas (Fig. 1B, locality 5), is mentioned inthe text. Two Callovian representatives, one recoveredfrom Quebrada El Asiento (Pérez, 1982, p. 60; fossillocality 10) and the other from Quebrada Palo Blanco

(east of Sierra Agua Amarga; Cornejo et al., 1993; Fig. 1,locality 1) are included in a sketch for comparison (Fig. 4).

Only few specimens of an undescribed species, herenamed Gryphaea n. sp. A, are known from Callovianbeds in Paso del Espinacito (Gottsche, 1878, 1925, Lám.4, fig. 12; Fig. 1B, locality 6). Other two probably co-spe-cific representatives were collected in Quebrada Ata-cameño (Rubilar, 1998; Fig. 1A, locality 4), whereOppeliidae, Macrocephalitidae, Reineckeiidae and Aspi-doceratidae ammonites are present (Callovian to Tithon-ian). Intensive tectonic activity makes dating these oys-ters difficult. In this paper, the general description ofGryphaea n. sp. A is based exclusively on the materialcollected by Gottsche (1878).

HETEROCHRONY IN A GROUP OF FIVE SOUTHAMERICAN MIDDLE JURASSIC GRYPHAEA SPECIES

The five species of Gryphaea here studied are regard-ed as a group of closely related oysters in a phylogeneticsense, because they share the following characters: 1, thegeneral configuration of the main part of the left valve(Fig. 2), especially in the first ontogenetic stages (pre-juvenile); 2, the comparable variability and/or recurrentpresence of similar structures (e.g. long subvertical andalmost flat anterior tilted surface; anterior flange and sul-cus; tubercle-like structures on some or all of the mostprominent zones of the shell; Table 1 and Figure 3); 3, therelatively low convexity and general integration of the pos-terior flange to the ventro-anterior growth of the main partof the shell (resulting in a more or less symmetric left valvein external view; Fig. 4); 4, the usual very small attachmentarea; and 5, the presence of thin radial striae on its externalsurface. This ornamentation (present in other taxa of Mid-dle Jurassic oysters) seemingly spreads up to a similarheight of the shell in the studied species.

Furthermore, most of the studied species werefound in one locality (Quebrada Las Varillas; Fig. 1A),in the same level or in different levels of a relativelyshort stratigraphical succession. Finally, they can bedistinguished from each other mainly by changes insize-shape relations (morphological development) ofsimilar characters, expressed along the ontogeny(Table 1; Figs. 3 and 4).

All these factors, including the most relevant mor-phological variability observed in the studied samples,enable the recognition of the five species mentionedbelow (three new species among them) and the estab-lishment of both a likely ancestor–descendant relation-ship and the probable size (morphology)–based hete-rochronic process involved in the ontogenetic changerecorded among them (Fig. 4, see below).





Main morphological features of the left valve

In the first ontogenetic stages of the studied species,the main part of the shell (Fig. 2) is prominent and gener-ally has marked changes in its antero-posterior convexityexpressed by a posterior and (especially) an anterior mar-gins (Fig. 2B), which are also the boundaries of the exter-nal surface of this zone (generally convex and symmet-ric). These margins are accompanied by a posterior and(especially) an anterior almost flat tilted surfaces, subver-ticals to the commissural plane (Figs. 2B and C).

In the following growth-stages, the anterior zone ofthe valve (near the commissural border) exhibits themost remarkable variability among the studied species(and in some of their representatives), where the ante-rior tilted surface and margin (of the main part of theshell) can be either absent or continue their growth inseveral ontogenetic pathways. In the last case, the mor-phological changes involve especially smoothing ortilting (in the anterior tilted surface) and widening orincreasing convexity (in the anterior margin). Theseontogenetic replacements usually begin around theheight where the valve has a marked antero-ventralcurvature (in external view). Contrary, in G. euteichan. sp. this curvature is not correlated with a change inthe development of the mentioned structures.

In most of the studied species, the earliest (or pri-mary) narrow anterior margin is replaced by a wideand/or strong surface. In two taxa (stratigraphicallyyounger G. oxytropis and G. euteicha n. sp.), it can bedeveloped as an anterior flange, which tends to be

emphasized generally both by locally prominentgrowth lines and by a more or less deep anterior sulcus(located on the external surface of the main part of thevalve; Fig. 4, specimens 4a, 5a, 7a, 8a; Figs. 5.7A, 5.7Cand 5.7D, 5.8C, 5.9B and 5.10).

Similarly, in the juvenile and towards the adultgrowth-stages, the earliest (or primary) small anteriortilted surface is replaced by a generally wider, slightlyconcave or convex and then not well defined surface.Nevertheless, it tends to be smooth and more or lesssubvertical to the commissure where the anteriorflange is prominent or even tuberculated (Figs. 4, spec-imens 4d, 7b, 8b, 9a and 9b; 5.7A and 5.7C, 5.8B,5.9A, 5.11A and 5.11B). In the adult shell of somespecimens (especially in G. euteicha n. sp.) the anteri-or and/or posterior tilted surfaces are named ‘walls’where they are more or less flat and placed at nearly rightangles to the mentioned plane (Figs. 5.7A, 5.8A and 5.9A).In the shells where the posterior flange lacks a well-definedconvexity (and is visible only in posterior view), the poste-rior tilted surface (or wall) can reach the posterior commis-sural border (Figs. 5.7B and 5.7D, 5.8C and 5.8D).

In an extreme variant, the earliest anterior margin isfollowed by a faintly convex and slightly tuberculatedsurface, and the anterior tilted structure is absent (Fig.4, specimen 11b and 11e).

The posterior margin of the main part of the shelltends to be the most convex (and narrow) zone of thevalve in the adult growth-stage (Figs. 2B and 2C), gen-erally named as ‘keel’.

FIGURE 2 Shell morphology of the genus Gryphaea and terminology of the left valve used throughout the text. A) and B) G. apiculata n. sp.,left valve, external and dorsal views respectively (based on SNGM 8586). C) G. oxytropis PHILIPPI, specimen with both valves together, rightvalve in external view (based on SNGM 8395).

BDevelopmental events recognized in the anterior commissure of the left valve Occurrence or comments

g Last development of the anterior tilted surface It generally undergoes strong changes along the ontogeny

h1. Low to moderate G. apiculata n. sp., G. oxytropis and G. varillasensis n. sp.h Antero-ventral curvature of the valve

h2. Very pronounced (or angular) G. euteicha n. sp. and Gryphaea n. sp. A

i Last development of the anterior convex margin Structure very reduced in Gryphaea n. sp. A (or flange)



The umbonal curvature (in anterior or posteriorview) can be either (1) absent (the apex points to somearea located over the hinge plane; Fig. 5.1C), (2) gentle(the apex points to a direction that is subparallel to thehinge plane; Figs. 5.2C and 5.4D), (3) moderate (theapex points to some area located between the hingeand commissural planes; Figs. 5.8B and 5.11D), or (4)strong (the apex points to the commissural plane orbeyond this, and the umbo has a curved profile; Fig. 4,specimen 5d).

Height and length of the left valve were mea-sured perpendicular and parallel to the hinge axis,respectively. The size, based on the height of the leftvalve, can be small (less than 30 mm), medium(between 30 and 60 mm) or large (more than 60 mm)

Developmental events in the left valve

The sequence of appearance/disappearance of sixmain characters or ontogenetic events (Table 1A), asfunction of size, was determined and measured fromthe apex to the ventral commissure along the most con-vex zone of the left valve (= peripheric growth/size;Fig. 3). This procedure is a lineal analysis becausedoes not consider the different umbonal curvature orbending among the specimens and/or species. The mostextreme differences are present in G. apiculata (absentor gentle umbonal curvature; Fig. 4, specimens 1c and2c) and in some Upper Bathonian – Lower CallovianG. oxytropis (strong umbonal curvature; Fig. 4, speci-men 5d). Nevertheless, most of the measured speci-mens have a moderate umbonal curvature.

ADevelopmental events measured in the middle zone of the left valve Occurrence or comments

a Posterior flange in external view

b Main convexity of the shell

c Last recognition of thin radial striae

d Anterior margin replaced by a flange

e Locally prominent structure on the main convexity of the shell

f Very curved, angular, ventral commissure

a1. Not visible

a2. Few or not apparent

a3. Progressively evident with growth

a4. Strong posterior expansion

b1. Relatively narrow and progressivelyprominent (‘keel’)

b2. Wide and low

b3. Abrupt replacement of the narrow main convexity by a wider surface

e1. Tubercle-like structure

e2. Well defined tubercle

G. apiculata n. sp.

G. euteicha n. sp.

G. oxytropis , G. varillasensis n. sp. and Gryphaea n. sp. A

G. apiculata n. sp., G. euteicha n. sp. and in some G. oxytropis

It is located in the posterior half of the valve in G. apiculatan. sp. and G. euteicha n. sp., in the middle of the shell inG. oxytropis and in the middle zone of the main part of thevalve in Gryphaea n. sp. A

It is higher in the middle zone of the valve in G. varillasensis n.sp.

This morphological change occurs especially in Gryphaea n.sp. A. It is also present in some specimens of G. oxytropis

Seemingly with similar spread in the studied species

G. oxytropis (especially younger representatives), G. euteichan. sp

G. apiculata n. sp. (some specimens), G. oxytropis (older rep-resentatives) and G. euteicha n. sp

G. oxytropis (younger representatives) and Gryphaea n. sp. A

Probably well defined in all the studied species

In the earliestgrowth stage

In later growthstages

TABLE 1 Main developmental events and range of variability detected in the left valve, measured in its middle zone (A) or recognized next tothe anterior commissure (B). Figure 3 shows the sequence of these ontogenetic events in each species.



The recognition of one character or event in eachspecies is only a comparative parameter (= point ofreference), because it does not imply that the characteror event has the same degree of development in allthese taxa, at the respective ontogenetic stage (seeTable 1).

The appearance/disappearance of other three char-acters or events, present next to the anterior commis-sural margin of the left valve (Table 1B), was correlat-ed with the sequence measured along the most convexzone based on the growth lines (Fig. 3).

The developmental events are graphically exempli-fied by the following and representative specimens: 6of G. apiculata n. sp. (SNGM 8581, 8587, 8591,8595); 7 of Late Early Bajocian – Late Bathonian G.oxytropis PHILIPPI (SNGM 8389, 8395, 8404, 8406,8409 - 8410, 8896) and 5 of Late Bathonian – LowerCallovian G. oxytropis PHILIPPI (SNGM 8367, 8371,8377, 8382 – 8383); 3 of G. varillasensis n. sp.(SNGM 8467 – 8468, 8470); 7 of G. euteicha n. sp.(SNGM 8438, 8512, 8607, 8615, 8622, 8633, 8636);and 1 of Gryphaea n. sp. A (DNGM 7943).

FIGURE 3 Comparative developmental events in five species of Middle Jurassic oysters as a funtion of size, measured in the periphery of theleft valve. In the horizontal axis the appearance/disappearance of the events are indicated, evaluated along the middle zone of the valve(main convexity of the shell). Below them the relative co-occurrence of other three events observed next to the anterior commissure are indi-cated, based on the growth lines. The ontogenetic sequence of the older and younger representatives of Gryphaea oxytropis PHILIPPI is shownseparately. Symbols refer to Table 1. Dotted lines connect the occurrence of two developmental events where the middle is not recorded.Plotted specimens are indicated in the text.

Morphology of the species and heterochronicrelationships

Gryphaea apiculata n. sp. Figures 2A and 2B; 4, specimens 1 and 2; 5.1 to 5.3 (see diagnosis in APPENDIX)

This species is of medium to small size (maximumheight ca. 39 mm). It was recorded from Middle Bajocianbeds at Quebrada El Asiento (Pérez, 1982, p. 43; Rubilar,1998; Fig. 1A), and is considered as the probable ances-tral stock to the group of oysters here analysed (Fig. 4).

Gryphaea apiculata n. sp. shows the following mostrelevant developmental events (Table 1; Fig. 3A): 1,appearance of the posterior flange, not visible in externalview (a1); 2, appearance of the relatively narrow and pro-gressively prominent main convexity of the shell, locatedin the posterior half of the valve from the earliest growth-stage (b1); 3, last development of the anterior tilted sur-face (g); 4, low to moderate antero-ventral curvature ofthe left valve (h1); 5, last development of the anteriorconvex margin (i); 6, probable last recognition of thinradial striae (c; there is a tubercle-like structure at thisontogenetic stage in some specimens); 7, strong posteriorexpansion of the posterior flange, in external view (a4).In some specimens, as the holotype, events 3 and 5 occurlater in the ontogeny.

In the first growth-stages, the main part of the shell hasanteriorly both a margin and a tilted surface extended tothe commissure (Figs. 2B and 5.1D). Generally, thesestructures are restricted to a very early stage, and endaround the height where the shell has a low to moderateantero-ventral curvature in external view (Figs. 3A and5.3B). The posterior margin of this area (and the relatedposterior tilted surface) is more conspicuous as the shellgrows up, and constitutes the main convexity of the valve,located in its posterior half (Figs. 2B, 3A, 5.1D and 5.3B).

In the pre-juvenile and juvenile growth-stages, the poste-rior flange (and sulcus) can be recognized only in posteriorview, while it is well defined and visible in external view lat-er in the ontogeny (Figs. 2A, 2B, 3A, 5.1B and 5.1C).

The umbo is opisthogyrous to orthogyrous, short, witha low convexity and gentle or absent curvature or bend-ing, projected very little above the commissural plane(Figs. 5.1C, 5.1D, 5.2B and 5.2C). The thin radial striaeare not well preserved; they probably cover most of theexternal surface of the valve. The ventral commissure isprobably very curved.

In some specimens of G. apiculata n. sp. the sulcusand posterior flange are visible in external view from theearliest growth-stage. In addition, the main convexity is

narrow, located in the middle part of the shell and bearstubercle-like structures (Figs. 4, specimen 2a and 2b;5.2A and 5.2B). This overall morphology precedes theanatomical development found in the adult specimens of G.oxytropis (compare Fig. 4, specimen 2 with 3 and 4), indicat-ing the phylogenetic relationships between both taxa.

Gryphaea oxytropis PHILIPPI, 1899Figures 2C; 4, specimens 3 to 6

Gryphaea oxytropis (Holotype: MNHN, SGO PI.5015) is generally of medium to large size (maximumheight ca. 72 mm). It has been recorded from Late EarlyBajocian to Middle Callovian beds in several localities inChile and Argentina (Rubilar, 1998; this paper). The onto-genetic changes that occur in the older (Late Early Bajo-cian – Late Bathonian) and some of the younger (LowerCallovian) representatives of this species are analysedhere, based on material collected especially at QuebradaLas Varillas (Fig. 1A) and Quebrada Asientos (Pérez,1982, p. 60), respectively.

The Late Early Bajocian – Late Bathonian specimensof G. oxytropis show the following most relevant deve-lopmental events (Table 1; Fig. 3B): 1, appearance of theposterior flange, progressively more evident with growth(a3); 2, appearance of the relatively narrow and progres-sively prominent main convexity of the shell, with a con-tinuous change of its position from the posterior half tothe middle zone of the valve (b1); 3, the anterior marginis generally replaced by an anterior flange (d); 4, lastdevelopment of the anterior tilted surface (g); 5, low tomoderate antero-ventral curvature of the valve (h1); 6,last recognition of thin radial striae (c); 7, first tubercle-like structure on the main convexity of the shell (e1); 8,very curved, angular, ventral commissure (f); 9, lastdevelopment of the anterior convex margin or flange (i).

On the other hand, in the Upper Bathonian – LowerCallovian specimens of G. oxytropis the most relevantdevelopmental events are (Table 1; Fig. 3C): 1, appear-ance of the posterior flange, progressively more evidentwith growth (a3); 2, appearance of the relatively narrowand progressively prominent main convexity of the shell,with a continuous change of its position from the posteri-or half to the middle zone of the valve (b1); 3, first well-defined tubercle on the main convexity of the shell (e2);4, the anterior margin is replaced by an anterior flange(d); 5, last recognition of thin radial striae (c); 6, low tomoderate antero-ventral curvature of the valve (h1); 7,last development of the anterior tilted surface (g); 8, verycurved, angular, ventral commissure (f; at this ontogenet-ic stage, in some specimens there is an abrupt replace-ment of the narrow main convexity of the shell by a widersurface; Fig. 4, specimen 4a); 9, last development of theanterior convex margin or flange (i).



In the first ontogenetic stages, the left valve of G.oxytropis is similar to the adult morphology of G. apicu-lata n. sp., considering especially the form and extensionof the anterior margin of the main part of the shell and theearliest position of its posterior margin (Figs. 2B and 2C;3A to 3C). On the other hand, in G. oxytropis the earlyand intermediate stages of growth are maintained longer,as shown by the larger adults of the later species (Figs.3B and 3C; 4, specimens 3 to 5) in comparison with G.apiculata n. sp. (Figs. 3A; 4, specimens 1 and 2).

This probable extension of the growth period in G.oxytropis, or hypermorphosis, results in several morpho-logical characteristics quite different from those of theancestral adult of G. apiculata n. sp. For example, theposterior margin of the main part of the valve (which con-stitutes its main convexity) is now located in the middlezone of the shell (compare Fig. 4, specimens 1b with 3b

and 4b); the earliest anterior margin (of the main part ofthe shell) not only is maintained and strengthened into theadult growth-stage (Fig. 3B, event i), but also is replacedby a well defined (and somewhere tuberculated) anteriorflange in stratigraphically younger (Late Bathonian – EarlyCallovian) specimens, generally accompanied both by ananterior tilted surface (which tends to be better defined inall growth-stages, along the geologic time; Fig. 3C,events g and i) and a sulcus (Fig. 4, specimens 4 and 5);and the umbo is generally strongly curved (Fig. 4, speci-mens 3c and 5d). Furthermore, the ventral commissuralmargin is more angular or very curved (e.g. Alvarez,1996, Fig. 36b) in comparison with G. apiculata n. sp.

The ontogenetically later antero-ventral curvature ofthe shell in G. oxytropis than in G. apiculata n. sp. (Fig.3A to 3C, event h) may be correlated with the greaterextension and strength of the anterior margin.



FIGURE 4 Size (morphology)-based heterochrony in five species of Gryphaea from Middle Jurassic of southern South America, exemplified byspecimens (1 to 11) representative of the most important variability observed. The relative position of specimens 3 and 6 to 10 is based onthe stratigraphical succession in Quebrada Las Varillas; 5 (Quebrada Palo Blanco), 11 (Paso del Espinacito) and especially 1 and 4 (Quebra-da El Asiento) are situated according to their age. Left valve, a: external view; b: dorsal view; c: posterior view; d: internal view. Right valve,e: external view. The figures are based on the following specimens: 1, SNGM 8586; 2, SNGM 8587; 3, SNGM 8395; 4, SNGM 8377; 5,SNGM 8458; 6, SNGM 8434; 7, SNGM 8606; 8, SNGM 8673; 9, SNGM 8607; 10, SNGM 8468; 11, DNGM 7943.

The older (Late Early Bajocian – Late Bathonian) re-presentatives of G. oxytropis, found in Quebrada Las Va-rillas, show the strongest resemblance with G. apiculatan. sp. in the first stages of growth (e.g. the form andextension of the anterior and posterior margins of themain part of the shell; Figs. 2B and 2C). On the contrary,the posterior flange (and sulcus) is generally convex (andvisible in external view) earlier in G. oxytropis than in G.apiculata n. sp. (Fig. 3A to 3C, event a; compare Fig. 4,specimens 1a and 3b), showing a probable pre-displace-ment process associated with the hypermorphosis in theorigin of the first taxon.

Gryphaea oxytropis can be interpreted as a polymor-phic species on account of its wide range of variability,especially in the dorso-ventral extension and convexity ofthe anterior margin/flange of the main part of the leftvalve, the related extension and width of the anterior til-ted surface (Fig. 3B and 3C, events g and i), and in thegeneral convexity of this anterior zone of the shell (Fig. 4,specimens 3 to 6). In this context, ‘Gryphaea tricarinataPHILIPPI (1899, p. 12 - 13, Lám. 5, fig. 1a - c), regarded as asynonym of G. oxytropis (Rubilar, 1998), represents a mor-photype with a well-developed anterior flange and sulcus.

These morphological differences during growth,observed in specimens collected at several localities,seem to be related with their stratigraphical positions, andhence they could represent an anagenetic change causedby a persistent pre-displacement heterochronic process.This is evidenced especially by the growth and positionof the tubercles in the main convexity zone (‘keel’), inaddition to the mentioned basic pre-displacement devel-opment of the sulcus and posterior flange.

Well-defined tubercles are absent in Late Early Bajo-cian - Late Bathonian specimens, although the ventral halfof the valve generally tends to have localized elevations inthe main convexity zone (Fig. 3B, event e1 and Fig. 4,specimens 3a and 6a). On the contrary, most of LateBathonian and Early Callovian specimens have moderateto large tubercles from the first growth-stages (Figs. 3C,event e2 and Fig. 4, specimens 4 and 5), and these mayalso be present on the anterior and posterior flanges. TheCallovian representatives also seem to have a narrower andearlier developed main convexity zone (Fig. 4, specimen 5)than Bathonian - Callovian specimens.

The size differences in mature representatives of G.oxytropis suggest tendencies to gigantism and dwarfism.The biggest known specimen of this species (height ca.128 mm) was collected by Alvarez (1997; CPBA 18146)in Lower Callovian beds of Arroyo las Garzas (San JuanProvince, Argentina; Fig. 1B). On the other hand, thesmallest mature specimens (maximum height ca. 40 mm;Fig. 4, specimen 6) come from Quebrada Las Varillas

(Fig. 1A), among others with a ‘normal’ height (ca. 70mm). They are regarded as adults because of the earlydevelopment of a strongly curved ventral commissure andthe growth of the valves focused subvertical to the com-missural plane in this zone (Fig. 4, specimen 6c). In thesespecimens the posterior flange is very narrow relative tothe common development in G. oxytropis (Fig. 4, speci-mens 3 to 5). This can be regarded as a ‘morphofunction-al limitation’. These dwarf specimens are considered tobe the direct predecessors of G. euteicha n. sp., withwhich they share the maximum size (compare Figure 4,specimens 6 with 7 to 9), among other characters.

Gryphaea varillasensis n. sp.Figures 4, specimen 10; 5.4 and 5.5 (see diagnosis in APPENDIX)

This species has a medium sized shell (maximumheight ca. 47 mm). It was recovered from Late EarlyBajocian - Late Bathonian units in Quebrada Las Varillas(Fig. 1A, levels V1 to V3).

Gryphaea varillasensis n. sp. shows the followingmost relevant developmental events (Table 1; Fig. 3D):1, appearance of the posterior flange, progressivelymore evident with growth (a3); 2, development of awide and low main convexity of the shell, located in themiddle zone of the valve (b2); 3, last development of theanterior tilted surface (g); 4, last development of theanterior convex margin (i); 5, low antero-ventral curva-ture of the valve (h1); 6, last recognition of thin radialstriae (c); 7, very curved (more or less angular) ventralcommissure (f).

Gryphaea varillasensis n. sp. and G. oxytropis have avery similar early development of the left valve in termsof the structure, width and almost symmetric convexity ofthe main part of the shell, the early appearance (in exter-nal view) of the posterior flange, the gentle or absentumbonal bending, and the type of thin radial striae (Figs.3B, 3D and 4, specimens 3e and 10b). Nevertheless, in G.varillasensis n. sp. this early development of the leftvalve is maintained until the mature stage of growth,where the most convex surface remains low, the externalsurface of the main part of the shell is very wide andalmost symmetric (without an anterior flange), the sulcus(narrow and relatively shallow) and posterior flange (lowconvex) tend to have a low development or to be integralwith the antero-posterior convexity of the valve, the umbomaintains gentle (or absent) curvature (with a very reducedconvexity; Fig. 4, specimen 10c and 10e), and the ventralcommissural margin may be only slightly curved.

Then, in comparison with the older (Late Early Bajo-cian – Late Bathonian) representatives of G. oxytropis, G.varillasensis n. sp. shows a marked ontogenetic delay in



the appearance of high shell convexity (e.g. absence of anarrow main convexity zone, an anterior convex marginor even a flange), whereas other characters tend to havethe same rate of development as in the mentioned repre-sentatives of G. oxytropis (especially the depth of the sul-cus and width of the posterior flange, the spread of thethin radial striae, and probably the length of the anteriormargin of the main part of the shell; compare Fig. 4, spe-cimens 3 and 10).

These differences in the rate of development of certainstructures, which are relatively retarded in G. varillasen-sis n. sp. with respect to its probable ancestor (G. oxytro-pis), would represent a heterochronic process of post-dis-placement. Moreover, some characters in G. varillasensisn. sp. are more comparable with G. apiculata n. sp., as isthe case with the lowest umbonal convexity and curvature(compare Fig. 4, specimens 2c and 10c). This apparent‘regression’ of G. varillasensis n. sp. to an ancestral mor-phology (showed by G. apiculata n. sp.; see Fig. 4) canbe interpreted as another evidence of the mentioned post-displacement process and, at least indirectly, supports theorigin of G. oxytropis (probably the direct ancestor of G.varillasensis n. sp.) from G. apiculata n. sp.

Nevertheless, the small size of G. varillasensis n. sp.relative to G. oxytropis can be interpreted as a partialoperative concurrence with progenesis, where the adultdescendant is smaller and retarded relative to the ancestor(although in progenesis the morphological change affectsthe whole organism).

Excluding the holotype (Figs. 5.5A to 5.5C), somespecimens have a shorter extension of the anterior tiltedsurface, followed by a low to moderate antero-ventralcurvature of the shell (Fig. 3D, events g and h1), and theanterior margin of the valve may have a convex outline(Figs. 4, specimen 10; 5.4B and 5.4C). This morphologyis probably related with the absence of a prominent ante-rior margin or flange (of the main part of the shell) in thejuvenile and adult growth-stages. Then, it would representan emphasized tendence to the mentioned delay in theappearance of a high shell convexity, in comparisson withthe Late Early Bajocian – Late Bathonian representativesof G. oxytropis.

Gryphaea euteicha n. sp.Figures. 4, specimens 7 to 9; 5.6 to 5.11 (see diagnosis in APPENDIX)

This species has small to medium size (maximumheight ca. 43 mm; generally less than 35 mm). It has beenrecorded from Late Bathonian and Callovian strata inQuebrada Las Varillas (Fig. 1A, levels V3 and V4) andQuebrada El Asiento (Pérez, 1982, p. 59-60; Fig. 1A;Rubilar, 1998), respectively. The ontogenetic changes that

occur in the older (Late Bathonian) representatives of thisspecies are analysed here.

In Quebrada Las Varillas, the upper levels with G.oxytropis (maximum height ca. 70 mm; Fig. 4, specimen 3)also bear smaller shells, which belong either to G. oxytro-pis (with a tendency to dwarfism; Fig. 4, specimen 6) or tothe earliest G. euteicha n. sp. (Fig. 1A, levels V3 and V4).

Some of the last mentioned specimens are consideredmorphologically ‘precursors’ (Figs. 4, specimen 7; 5.11),with a transitional morphology between G. oxytropis andthe most common representatives of G. euteicha n. sp.(Fig. 4, specimens 8 and 9). On the other hand, this earliestevolutionary stage of G. euteicha n. sp. probably representsthe ancestral stock of other South (and North) Americanlate Middle Jurassic species, which share the general pat-tern of G. euteicha n. sp. left valve although their variabili-ty includes, at least, three different morphotypes.

Typical representatives of G. euteicha n. sp. show thefollowing most relevant developmental events (Table 1;Fig. 3E): 1, appearance of the posterior flange, little ornot visible in external view (a2); 2, origin of the relativelynarrow and progressively prominent main convexity ofthe shell, located in the posterior half of the valve (b1); 3,very pronounced (angular) antero-ventral curvature of thevalve (h2); 4, the anterior margin is replaced by an anteri-or flange (d); 5, last recognition of thin radial striae (c); 6,strong posterior expansion of the posterior flange (a4); 7,very curved, angular, ventral commissure (f); 8, last develop-ment of the anterior tilted surface or wall (g); 9, last develop-ment of the anterior convex margin or flange (i).

In the first (‘precursor’) specimens of G. euteicha n.sp., the posterior flange has a low and uniform convexity,at least, in a young growth-stage (dorsal and middle partof the shell), resembling dwarf specimens of G. oxytropis(Fig. 4, specimen 6), where it is generally very narrow inexternal view (in some specimens it may be wider just atmaturity, near the ventro-posterior margin). In the men-tioned specimens of G. oxytropis the posterior flange gen-erally is, however, wider throughout the ontogeny, and isusually well defined in external view from its origin.

Gryphaea euteicha n. sp. and G. oxytropis have a verysimilar early development of the left valve in width andconvexity (at first almost symmetric and then asymmet-ric) of the main part of the left valve, the early origin ofthe posterior flange (Figs. 3B and 3E, events a and b1)and the gentle or absent umbonal curvature (compare Fig.4, specimens 3b and e with 7b and 8b).

Stratigraphically, the mentioned low development ofthe posterior flange in G. euteicha n. sp. is very marked,and in the younger specimens this structure is very expand-



ed only in the adult growth-stage (Fig. 3E, event a4) oreven just visible in posterior view (Figs. 4, specimens 8and 9; 5.6A and 5.6B, 5.7B to 5.7D, 5.8C and 5.8D), simi-lar to some extent to G. apiculata n. sp. (Fig. 4, specimen1a to c). According to this trend, the posterior flange has aretarded growth relative to G. oxytropis (Fig. 4, specimens3a and 6a), exhibiting a post-displacement process.

In the earliest and most of the later (typical) represen-tatives of G. euteicha n. sp., the main convexity of theshell is located near the posterior margin of the left valve(Figs. 4, specimens 7b and 8b; 5.6B, 5.7A and 5.11A), asin G. apiculata n. sp. (Fig. 4, specimen 1b) and in the firstgrowth-stages of G. oxytropis (Fig. 4, specimen 3e),whereas it is placed in the middle part of the adult shell inthe last species (Fig. 4, specimens 3b, 4b, 5a and 6a). Inother words, G. euteicha n. sp. has morphological charac-ters of maturity (the position of the main convexity of theshell) similar to those present only in the first growth-stages in G. oxytropis.

On the contrary, other structures of the left valvehave an equivalent development throughout the ontoge-ny in both species (e.g. the general prominent and nar-row main convexity of the shell, the presence of theanterior margin or flange and a tilted surface in the firststages of growth or along the ontogeny, the extension ofthe thin radial striae, the moderate to strong umbonalcurvature, and the curvature of the ventral commissuralmargin; Fig. 3B and 3E, events b1, f to i), as it is a char-acteristic in the post-displacement heterochronic processhere proposed.

The specimens of G. euteicha n. sp. from the upperlevels in Quebrada Las Varillas have new and remarkablecharacters, especially in the antero-ventral zone of theshell. The left valve tends to have a pronounced antero-ventral expansion, originated from an early curvature inthe commissural margin (Figs. 4, specimen 9; 5.6B and5.8A to 5.8C), the anterior tilted surface (long and gener-ally high (= anterior wall; Figs. 5.7A and 5.7C, 5.8B and5.8C, 5.9A and 5.9B) and the anterior margin (or flange)are well developed along the growth, and the antero-pos-terior convexity of the shell (Fig. 5.8E) tends to be evenly

symmetric (Figs. 4, specimen 9a, b; 5.8A and 5.8C).These characters suggest a very accentuated anageneticpost-displacement process.

Gryphaea n. sp. AFigure 4, specimen 11

The following description of the ontogenetic changesthat occur in this taxon is based on the material collected byGottsche (1878, 1925, Lám. 4, Fig. 12; DNGM 7943) fromCallovian levels at Paso del Espinacito (Fig. 1B). They areof medium to large size (maximum height ca. 71 mm). Oth-er two large specimens (maximum height ca. 113 mm), notwell preserved and collected in Quebrada Atacameño (Rubi-lar, 1998; Fig. 1A), are also assigned to this species.

The best-preserved specimen (Gottsche, 1878, Lám.4, fig. 12; fig. 4, specimen 11) shows the following mostrelevant developmental events (Table 1; Fig. 3F): 1,appearance of the relatively narrow and progressivelyprominent main convexity of the shell, located in the mid-dle zone of the main part of the valve (b1); 2, appearanceof the posterior flange, progressively more evident withgrowth (a3); 3, first tubercle on the main convexity of theshell (e2); 4, last development of the anterior tilted sur-face (g); 5, last recognition of thin radial striae (c); 6,very pronounced (angular) antero-ventral curvature of thevalve (h2); 7, strong posterior expansion of the posteriorflange (a4); 8, abrupt replacement of the narrow mainconvexity of the shell by a wider surface (b3); 9, lastdevelopment of the anterior margin (i); 10, very curved,angular, ventral commissure (f).

Gottsche’s specimen could be considered morphologi-cally close to older and younger representatives of G.oxytropis, here described. Despite the few available speci-mens and limited biostratigraphic information onGryphaea n. sp. A (and the presence of a slightly tubercu-lated ‘anterior margin’ of the main part of the shell, in theanalysed specimen; see below), the development of theanterior commissural zone and especially the absence ofan anterior flange allow to consider the mentioned speci-men as closely related to the older (Late Lower Bajocian– Upper Bathonian) representatives of G. oxytropis.



FIGURE 5 1 to 3. Gryphaea apiculata n. sp. Quebrada El Asiento, Middle Bajocian. 1) Holotype, SNGM 8586, left valve. 1A: anterior view;1B: external view; 1C: posterior view; 1D: dorsal view. 2) Paratype, SNGM 8587, left valve. 2A: external view; 2B: dorsal view; 2C: posteriorview. 3) Paratype, SNGM 8589, left valve. 3A: external view; 3B: dorsal view; 3C: posterior view.

4 and 5. Gryphaea varillasensis n. sp. Quebrada Las Varillas, Late Early Bajocian - Late Bathonian. 4) Paratype, SNGM 8468, specimenwith both valves. 4A: left valve, dorsal view; 4B: left valve, external view; 4C: right valve, external view; 4D: left valve, posterior view. 5) Holo-type, SNGM 8467, specimen with both valves. 5A: left valve, dorsal view; 5B: left valve, external view; 5C: right valve, external view.

6 to 11. Gryphaea euteicha n. sp. Quebrada Las Varillas, Late Bathonian. 6) Paratype, SNGM 8633, left valve. 6A: posterior view; 6B:external view. 7) Paratype, SNGM 8622, specimen with both valves. 7A: left valve, dorsal view; 7B: left valve, posterior view; 7C: left valve,anterior view; 7D: left valve, external view. 8) Holotype, SNGM 8607, left valve. 8A: dorsal view; 8B: anterior view; 8C: external view; 8D:posterior view; 8E: internal view. 9) Paratype, SNGM 8615, left valve. 9A: anterior view; 9B: external view. 10) Paratype, SNGM 8636, leftvalve, external view. 11) SNGM 8606, left valve. 11A: dorsal view; 11B: anterior view; 11C: external view; 11D: posterior view.

The mentioned specimen shares with the older G.oxytropis the general morphology of the main part of theleft valve (e.g. the form of its anterior margin and espe-cially the short ontogenetic extension of the anterior tilt-ed surface) and the early appearance of the posteriorflange (and sulcus) in the first growth-stages (Figs. 3Band 3F, events a, g, i).

Nevertheless, in Gottsche’s specimen the prominentand narrow main convexity of the valve occurs earlierand has well defined tubercles (Figs. 3F, event e2 and 4,specimen 11b). In addition, the almost flat and slightlytuberculated surface or zone, present in a pre-juvenilegrowth-stage, that replaces the earliest anterior convexmargin of the main part of the shell (Fig. 4, specimen11e), seems to be equivalent to the strong margin gener-ally present towards the adult stage in the mentioned G.oxytropis (Fig. 4, specimen 3a and 3e). It is remarkablethat both structures constitute the anterior commissuralborder of the shell, in absence of the anterior tilted sur-face at these growth-stages.

This appearance of structures at an earlier growth-stage in Gryphaea n. sp. A relative to late Lower Bajo-cian – Upper Bathonian G. oxytropis, resulting in a mor-phologically more advanced (and larger) left valve, canbe related with a pre-displacement heterochronicprocess.

On the other hand, in the specimen of Gryphaea n.sp. A, the ontogenetic events towards the adult growth-stage include a very pronounced antero-ventral (andposterior) curvature (and expansion) of the shell (Fig.3F, event h2). In addition, the narrow main convexityof the shell (located in the middle portion of the mainpart of the valve from the first growth-stages) isreplaced by a wider convex surface (Fig. 3F, event b3),the sulcus is broad, and in the adult stage the valve hasalmost subparallel margins (Fig. 4, specimen 11). Onthe contrary, in late Lower Bajocian – Upper Bathon-ian G. oxytropis the antero-ventral curvature of theshell is low to moderate, the main convexity of theshell is generally narrow and is always located in themiddle part of the left valve (as a whole), the sulcus isdeeper than wide (Fig. 4, specimens 3 to 6), and theshell has a subtriangular outline.

These morphological differences in the adult shell ofGryphaea n. sp. A and G. oxytropis, and the greater sizeof the valve in the first taxon (considering especially theprobable cospecific specimens collected in QuebradaAtacameño), show a partial operative concurrence withhypermorphosis, where the adult descendant (Gryphaean. sp. A) is larger and differs from the ancestor(although in hypermorphosis the morphological changeaffects the whole organism).

DISCUSSION AND CONCLUSION

The significance of G. apiculata n. sp. as the ancestorof the group of South American Middle Jurassic oystershere studied is supported by its stratigraphical positionand because most of its external left valve anatomy andmorphological development can be more or less recog-nized in all the other species here described (Fig. 4): 1,the earliest configuration of the main part of the shell(especially the presence of an anterior tilted surface andmargin), including the form of the umbonal apex and thegenerally very small attachment area; 2, the general sym-metric outline of the valve; 3, the presence of an antero-ventral curvature; 4, the relatively low convexity and adultexpansion of the posterior flange, approximately at theheight where the antero-ventral curvature occurs; 5, the ten-dency to have an asymmetric ventral commissure; and 6,probably the nature and extension of the thin radial striae.

Excepting G. varillasensis n. sp., all the studied specieshave, at least, a tendency to develop tubercle-like structureson some (or all) the most prominent zones of the valve(main convexity and flanges). This is regarded as anotherevidence for the mentioned close morphological relation-ships, because its absence in any ontogenic stages in G.varillasensis supports the supposedly ‘retarded’ develop-ment of this species in comparison with G. oxytropis.

Gryphaea oxytropis and G. euteicha n. sp. have thestrongest morphological affinities with G. apiculata n.sp., based especially on the tendency to have a prominentanterior commissural border in the adult shell, as a resultof the co-occurrence of a well defined anterior tilted sur-face and margin.

In G. oxytropis and G. euteicha n. sp. this anterior tilt-ed surface tends to be particularly well defined alongontogeny and the related margin can be reinforced by ananterior flange, showing that they are closely related. Infact, G. euteicha n. sp. probably descends from dwarfrepresentatives of G. oxytropis, along a progressive ana-genetic process (Fig. 4). The mentioned similar develop-ment in the anterior commissural border, particularly bestexpressed along time in both species, could be interpretedas a parallelism, although this growth actually indicatestheir common ancestor. On the other hand, the origin ofG. oxytropis from G. apiculata n. sp. is also evidencedbecause in the Middle Bajocian population of the latespecies a morphotype strongly similar to the first LateEarly Bajocian – Late Bathonian G. oxytropis is present(compare Fig. 4, specimens 2, 3 and 4).

Gryphaea euteicha n. sp. is the descendant speciesmorphologically most similar to G. apiculata n. sp.,resembling its main ontogenetic development. They shareespecially the tendency to have a wide and almost flat



external surface of the left valve, the delayed emergenceof the posterior flange and the posterior position of themain convexity of the shell (which concurred to the develop-ment of a more or less defined posterior tilted surface or wallin the valve), and probably its size. In the context of thiswork, this affinity reflects a post-displacement heterochronicprocess that explains this apparent ‘regression’ to an ances-tral morphology and, at least indirectly, supports the originof G. oxytropis (probably the direct ancestor of G. euteichan. sp.) from G. apiculata n. sp. (Fig. 4).

This origin of G. oxytropis from G. apiculata n. sp.,and the significance of the last species as the ancestor ofthe taxa here studied, is also indirectly supported by theresemblance between G. varillasensis n. sp. (probablyoriginated from G. oxytropis) and G. apiculata n. sp.,considering the lowest umbonal convexity and curvature(Fig. 4, specimens 2c and 10c).

Gryphaea oxytropis shows the highest morphologicalvariability, which seems to be closely related to severalheterochronic processes. It is evidenced by the probableanagenetic change involved in its evolutionary history,and especially by the fact that the older specimens arerelated to the origin of, at least, two sympatric species ina relatively short period of time (Fig. 4). The apparentanagenetic tendency here mentioned, based, for example,on variations of the convexity of both main part of theshell and anterior margin (and flange), must be studied indetail with a more accurate biostratigraphic record.

In summary, all these strong morphological affinities,as well as the ontogenetic differences in relative timeapparition and development of several characters (e.g.margins, sloped surfaces, antero-ventral curvature andconvexity of the main part of left valve; main convexityof shell and tubercle-like structures; posterior and anteriorflanges and sulcus; curvature of the umbo; size change ofthe shell), are interpreted as the products of heterochronicprocesses, involving the cladogenetic origin of the olderG. oxytropis (Late Early Bajocian – Late Bathonian) fromG. apiculata n. sp. (Middle Bajocian), and G. varillasen-sis n. sp. (Late Early Bajocian – Late Bathonian), G. eute-icha n. sp. (Late Bathonian – Callovian) and Gryphaea n.sp. A (Callovian) from the earliest representatives of G.oxytropis (Fig. 4). On the other hand, these heterochronicprocesses involves anagenetic variations in G. euteicha n.sp. and probable G. oxytropis.

These size (morphology)-based heterochronies maybe considered misleading, because changes in rate andtiming of ontogenetic events can, by definition, only bedetermined where the ontogenetic age of compared speci-mens is known (e.g. McKinney and McNamara, 1991;Jones and Gould, 1999). Nevertheless, their detection andevident correspondence with schemes based on allometric

manifestations (size-based heterochronies), described byseveral authors (see McKinney and McNamara, 1991,figs. 2-6), are valid and informative results at this descrip-tive level of analysis. They show the high importance ofheterochrony as the paleobiological mechanism involvedin the origin and variability of these oysters, and providehypotheses to be tested including the ontogenetic age ofeach compared specimen. If the rate of change in bodysize results similar between compared (closely related)species, as is suggested by the seemingly similar spreadof radial striae, the processes here recognized may reflectthe ‘true’ heterochronies involved.

The group of five species analysed represents themost remarkable example of iterative heterochronyknown to date in oysters, where several characters andmorphologically well-defined ontogenetic variations sup-port the ancestor-descendant relationships among them.The most-known and discussed example involving threespecies of Gryphaea from Hettangian to Pliensbachian inEngland has been classically considered an example ofphyletic size increase with a paedomorphic (neotenic)relationship (see Jones and Gould, 1999, and referencesthere). Although this heterochronic process was recentlyassessed by comparing standardized specimens of com-mon age (Jones and Gould, 1999), they may not representa unique lineage considering unpublished data fromSouth America (Rubilar, 1998).

ACKNOWLEDGEMENTS

The author is grateful to S. Damborenea and M. Manceñido(Museo de La Plata, Argentina) for their encouragement at allstages of this work. The manuscript was greatly improved byreviews of S. Damborenea, G. Westermann (Canada) and A.Riccardi (Museo de La Plata). E. Pérez (Sernageomin, Chile)fostered the study of Upper Triassic and Jurassic oysters, pro-vided regional geological insights and gave generous collabora-tion during fieldwork in northern Chile. S. Damborenea, A. Ric-cardi and A. Spotorno (Facultad de Medicina, Universidad deChile) provided valuable literature about heterochrony. M.Azpelicueta (Museo de La Plata) stimulated the beginning of thisstudy with a paper on the same subject. S. Damborenea, S.Matthews and P. Hofer (Sernageomin) reviewed the Englishgrammar of different versions of the text. The photographs weretaken at the Museo de La Plata by F. Castets (1997). S. Ross, N.Espinoza (Sernageomin) and R. Molina (Departamento deGeología, Universidad de Chile) helped with the figure design.

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A. RUBILAR

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Manuscript received August 2003;revision accepted November 2004.

SYSTEMATIC PALEONTOLOGY

Superfamily: Ostreoidea RAFINESQUE, 1815Family: Gryphaeidae VYALOV, 1936

GENUS Gryphaea LAMARCK, 1801

Gryphaea apiculata n. sp.Figures 2A and 2B; 4, specimens 1 and 2; 5.1 to 5.3

Etymology: apiculus (Latin): pointed, sharp, referingto the apex form.

Material: Holotype: SNGM 8586 (Fig. 5.1).Paratypes: SNGM 8581 - 8582, 8587, 8589, 8591,

8595.

Type locality and age: Quebrada El Asiento (26º23’S;69º23’W), Chile (fossil locality No. 11 in Pérez, 1982, p.43). Middle Bajocian.

Diagnosis: Medium to small (maximum height ca. 39mm) and subtriangular to subtrapezoidal (in some casessubovoidal) Gryphaea. In the dorsal third, the main partof the shell has an anterior margin and a tilted surface(extended to commissure). The posterior margin of thiszone (and the associated posterior tilted surface) is orient-ed in a ventroposterior direction along growth, and con-

stitutes the most convex surface of the shell, generallylocated in the posterior half of the valve. The posteriorflange is visible in external view only in the ventral por-tion of the shell and has a low convexity. The umbo isopisthogyrous to orthogyrous, with a low convexity, agentle or absent curvature and a short and sharp apex.External thin radial striae are not well defined. The shellhas a medium thickness.

Discussion: G. apiculata n. sp. shares with G. varil-lasensis n. sp. and G. euteicha n. sp. the earliest (primary)conformation of the main part of the shell, with a posteri-or and specially an anterior margins and a more or lessconvex (symmetric) external surface. Moreover, in G.varillasensis n. sp. the umbo has a low convexity and agentle or absent curvature, as in G. apiculata n. sp.

Nevertheless, this species has close morphologicalaffinities with G. euteicha n. sp., considering the general-ly wide and almost flat external surface of the left valve,the delayed emergence of the posterior flange and theposterior position of the main convexity of the shell. Onthe contrary, G. apiculata n. sp. differs from G. euteichan. sp. because in the last species the anterior tilted surfaceis usually well defined and high (= anterior wall) alongthe ontogeny, the antero-ventral curvature (and the relatedexpansion of the shell) is more pronounced (and does notaffect the development of the anterior wall), the anterior

APPENDIX

Diagnosis of new species

margin of the main part of the shell tends to be replacedby an anterior flange with the growth, and the umbo isgenerally more curved.

Gryphaea apiculata n. sp. differs from G. varillasen-sis n. sp. because in the latter the antero-ventral curvatureof the shell is generally lower (and the anterior margin ofthe valve can be uniformly convex), the anterior tiltedsurface and margin (of the main part of the shell) arerestricted to the earliest growth-stage, the main convexityof the left valve is lower, symmetric, and located in themiddle part of the shell, and the posterior flange becomesprogressively more evident with growth.

Gryphaea varillasensis n. sp.Figures 4, specimen 10; 5.4 and 5.5

Etymology: It refers to the type locality.

Material: Holotype: SNGM 8467 (Fig. 5.5).Paratypes: SNGM 8468 - 8470.

Type locality and age: Quebrada Las Varillas(26º26’S; 69º11’W), Chile. Late Early Bajocian - LateBathonian.

Diagnosis: Medium (maximum height ca. 47 mm)and subovoidal to subcircular Gryphaea. The main part ofthe left valve is delimited by rounded anterior and poste-rior margins only near the apex of the umbonal zone. Thegeneral convexity of the shell is low and wide, alsoreduced near the anterior commissure, and higher in themiddle part of the valve. The posterior flange is as wideas long during growth, and has a low convexity. Theumbo is opisthogyrous to orthogyrous, few convex, witha gentle (or absent) curvature and a short apex. Externalthin radial striae are well defined in the dorsal and middlethird of the shell. The ventral commissure is gently orvery curved in the adult growth-stage. The shell has amedium to low thickness.

Discussion: G. varillasensis n. sp. shares with G.apiculata n. sp. and G. euteicha n. sp. the earliest (prima-ry) conformation of the main part of the shell, with a pos-terior and specially an anterior margins and a more or lessconvex (symmetric) external surface. Moreover, in G.apiculata n. sp. the umbo has a low convexity and a gen-tle or absent curvature, as in G. varillasensis n. sp.

This species differs from G. apiculata n. sp. because inthe latter the antero-ventral curvature of the shell is low tomoderate, the anterior tilted surface and margin (of the mainpart of the shell) can be well defined in the adult growth-stage, the main convexity of the shell is high, narrower andlocated in the posterior half of the valve, and the posteriorflange is visible in external view later in the ontogeny.

Gryphaea varillasensis n. sp. differs from G. eute-icha n. sp. because in the latter the left valve is generallynarrow, the antero-ventral curvature of the shell is verypronounced, the anterior tilted surface and margin (orflange) of the main part of the shell are well developedalong the ontogeny, the main convexity of the shell ishigher (and narrow) from the first growth-stages andusually is located in the posterior half of the valve, theposterior flange is visible or well recognized in externalview only in later growth-stages, and the umbo is moreconvex and curved.

Gryphaea euteicha n. sp.Figures 4, specimens 7 to 9; 5.6 to 5.11

Etymology: eu (Greek): well; teichos (Greek): wall,refering to the well defined anterior and posterior tiltedsurfaces (or walls) of the main part of the shell.

Material: Holotype: SNGM 8607 (Fig. 5.8).Paratypes: SNGM 8435, 8440, 8612, 8615 - 8616,

8622, 8626, 8633, 8636, 8641 - 8642, 8665, 8670, 8673,8720 - 8721.

Type locality and age: Quebrada Las Varillas(26º26’S; 69º11’W), Chile. Late Bathonian. Other local-ity: Quebrada El Asiento, ca. 6.1 km to west of EstaciónMontandon; Pérez, 1982, p. 59-60), Chile. Callovian.

Diagnosis: Small to medium (maximum height ca. 43mm) and subovoidal - subrectangular to subtrigonalGryphaea, with a tendency to have a concave anteriormargin. The main part of the left valve is very prominent,delimited by narrow anterior and posterior margins,accompanied by anterior and posterior subvertical andflat surfaces (or walls) extended to the commissure. Theanterior margin tends to be replaced by an anterior flangealong the ontogeny, emphasized by an anterior sulcus(located on the external surface of the valve). The mainconvexity of the shell is located on its posterior half. Theposterior flange tends to be not visible in external view,integrated to the posterior surface (or wall) of the valve.The umbo is orthogyrous to gently opisthogyrous or pros-ogyrous, with a moderate to low convexity, generallywith a gentle or absent curvature (in some cases moderateto high) and a long apex. The thin radial striae are welldefined in the dorsal third of the shell. The anterior com-missure is more curved than the posterior one. Theumbonal cavity is deep. The shell is generally thick tovery thick.

Discussion: G. euteicha n. sp. shares with G. apicula-ta n. sp. and G. varillasensis n. sp. the earliest (primary)conformation of the main part of the shell, with a posteri-or and specially an anterior margins and a more or lessconvex (symmetric) external surface.



This species has close morphological affinities withG. apiculata n. sp., considering the generally wide andalmost flat external surface of the left valve, thedelayed emergence of the posterior flange and the pos-terior position of the main convexity of the shell. Onthe contrary, G. euteicha n. sp. differs from G. apicula-ta n. sp. because in the latter the anterior tilted surfacehas a lower degree of development and generally isrestricted to the first growth-stages, the antero-ventralcurvature is low to moderate and usually is present atthe same height where the anterior tilted surface andmargin (of the main part of the shell) have their lastoccurrence, the mentioned anterior margin never is

replaced by an anterior flange, and the umbo generallyhas a lower curvature.

Gryphaea euteicha n. sp. differs from G. varillasensis n.sp. because in the latter the left valve is generally wider alongthe growth, the antero-ventral curvature of the shell usually isvery reduced (and the anterior margin of the valve may have aconvex outline), the anterior tilted surface and margin (of themain part of the shell) are not well developed because they arerestricted to the earliest growth-stage, the main convexity ofthe shell is low, wider and is located in the middle part of thevalve, the posterior flange is progressively evident withgrowth, and the umbo has a lower convexity and curvature.



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